There are a few different guides to leveling locations. But this is my own personalized one, for good exp locations for parties. I keep finding really good locations, but then forgetting them!
This guide is not meant as "places you 'can' level with a party", but "Places you will be getting Rockin 200+ exp every time" type places, so long as ALL your party members are within the given level range.
36-38: Crawler's nest, worker crawlers 40ish: gustav tunnel, Bats 42-44: beetles in east altepa, near Revalation Rock 44-46: quicksand caves in EAST altepa 48-50: quicksand caves in WEST altepa? 48-51: Garleige basement, Beetles (G1 only from bombs though) 48-51: Crawlers Nest, Rumble crawlers and exorays(G1) (48-51: in theory, liches in garliege: G1 paper) 51-52: Kuftal Tunnel, crabs 52-54(?) Boyahda tree, crabs 54-56: Cape terrigan on beach 57-60: valley of sorrows: lizards, then bigugly birds 62-64: "sky" (flamingos) 63-70 bibiki bay (misc)